In today's competitive environment, a cellular system operator needs to carry as much traffic as possible on his spectrum without degrading service quality. Furthermore, an operator needs to provide coverage over large areas with modest traffic, as well as smaller areas with very high localized traffic, often referred to as “hot spots”. This is traditionally solved by having an hierarchical or overlay/underlay cell structure wherein a number of smaller pico or micro cells are provided within the coverage area of a larger macro cell. By providing handover between the cell structures, the small cells will only be needed where traffic demands are very high, or where the coverage from a macro cell is marginal, for example, in some indoor sites such as airports, railway stations and office buildings.
Traditionally, operators employing overlay/underlay cell infrastructures, such as that shown in FIG. 1, use different radio communication carriers for each cell layer in order to reduce mutual interference between the cell layers. This a natural technique for operators of more narrow band cellular standards, as a relatively large number of carriers are available to each operator. However, assigning different carrier pairs is difficult to do when an operator only has two or three carrier pairs available, as can be the case in a wide-band CDMA (WCDMA) system. As a result, operators with a limited number of carriers allow the lower layers or pico cells to use some or all of the uplink and downlink carrier pairs assigned to the upper or macro cell. Therefore, operators employ one of the following two techniques, (A) they assign different uplink and downlink carrier pairs to the pico and macro cell or (B) they allow the pico cell to use some or all of the uplink downlink carrier pairs assigned to the macro cell.
One problem with using different up-link/down-link carrier pairs in each cell layer is that it can result in inefficient use of the spectrum. For example, if an operator only has access to a few carrier pairs, for example, 2, they are forced to decrease the capacity of their macro cell by 50 percent in order to load a lower/pico cell using a different carrier pair. As this is typically unacceptable to the operator, operators of these system typically reuse the same carrier pair in both layers of the infrastructure. This is feasible as long as the traffic in the underlay cell is low.
However, the underlay cell traffic can interfere with the macro cell traffic and can, with increasing cell traffic, gradually reduce the capacity of the macro cell beyond an acceptable level. Then, from a capacity point of view, the operator ends up with concept A, i.e., the carrier pair being used by the underlay cell will more or less be useless for the macro cell. One method for improving the utilization of broad band RF carriers for overlay/underlay infrastructures in such systems and generally for asymmetric broadband data services is provided in copending application Ser. No. 10/233,663, titled “Flexible Carrier Utilization” (published as U.S. Patent Application Publication No. US 2003/0109284 A1 on Jun. 12, 2003), which is hereby incorporated by reference in its entirety.
The technique of the copending application improves the utilization of broad band RF carriers by allocating an extra frequency band, for example, an extra up-link, to the underlay cell as opposed to allocating a frequency pair. However, such a system solution needs a mobile terminal able to work with variable duplex, i.e., an unpaired UL and DL, when necessary.
In an asymmetric cellular system, for example WCDMA, the link from a mobile terminal to a base station, i.e., the uplink (UL) and the link from the base station to the mobile terminal, i.e., the down-link (DL), differs in terms of modulation, slot format, interleaving and coding. Furthermore, the frequency distance between the UL and DL is constant for all UL and DL pairs (i.e, they work in a fixed duplex mode). This means that the up-link and down-link frequency bands have a fixed separation in frequency, and if one of the links is moved to another frequency the other one is also moved to keep the fixed separation in frequency This can give rise to frequency spectrum utilization problems, as discussed above, since a change in the up-link will result in a change in the down link and vice versa. However, as discussed above, a capacity problem may exist in the uplink but not in the downlink. In other scenarios it can be the DL that has a capacity problem first while the UL is able to handle the current traffic. Therefore, there is a need for a method and apparatus able to solve the problem of one link with a telecommunication system having capacity problems while the corresponding link does not.